Releasable pressure sensitive adhesive composition

Abstract: A releasable pressure sensitive adhesive composition comprising: (a) a block copolymer comprising at least one poly(monovinyl aromatic hydrocarbon) block and at least one poly(conjugated diene) block and having a vinyl content of at least 3% by weight, (b) a tackifying resin, and (c) a plasticizer, wherein the block copolymer has been cross-linked through its vinyl groups by means of a reaction with an organohydrogenpolysiloxane crosslinking agent containing at least two hydrogen atoms being directly bonded to a silicon atom in the presence of a crosslinking catalyst and wherein the plasticizer is present in an amount of from 1 to 50 parts by weight per 100 parts by weight of block copolymer. The releasable adhesive composition can be suitably applied in removable self-adhering memo notes and in removable tapes and labels.

Description: The present invention relates to a releasable, pressure sensitive adhesive composition. More specifically, the present invention relates to a releasable, pressure sensitive adhesive composition comprising a cross-linked block copolymer based on a conjugated diene and a vinyl aromatic hydrocarbon. Releasable pressure sensitive adhesives are used in numerous applications wherein tackiness as well as removability without leaving traces of adhesives on the substrate -even after repeated use- are required. Specific examples include self-adhering memo labels, removable tapes and reusable closure systems, such as closures of packages containing tissues. Several adhesive compositions meeting the above requirements have already been proposed in the past. For instance, US-A-4,680,333 discloses a removable pressure sensitive hot melt adhesive composition comprising an ABA type block copolymer, with A being polystyrene and B polyisoprene, and a substantial amount (up to 50% by weight of the composition) of aliphatic hydrocarbon resin having a softening point of less than 30°C. Such hydrocarbon resins mainly include liquid tackifying resins. In addition to these components other components, such as high softening point aliphatic hydrocarbon resins, i.e. solid tackifying resins, mineral oil or petroleum, metallic salts of fatty acids and antioxidants, may be present too. WO 88/01631 and WO 88/01636 both relate to a readhering and releasable gelled solid adhesive for coating substrates, particularly paper, comprising a rubber component, a water-soluble or water-dispersible tackifying agent and a gelling agent. A friction reducing agent comprising a polyhydric aliphatic alcohol may be present as well. Natural rubber latex is stated to be the preferred rubber component. Preferred gelling agent is sodium stearate. EP-A-0,443,263 discloses a repositionable adhesive coated substrate whereby the adhesive layer comprises 20 to 80% by weight of an elastomeric styrene/butadiene, styrene/isoprene or styrene/ethylene-butylene block copolymer, the balance up to 100% by weight being a tackifier material. This tackifier material is either a tackifying resin (liquid and/or solid) or a blend of a tackifying resin and a liquid plasticizer oil. The purpose of the tackifier material is to modify the composite midblock glass transition temperature (CMTg), which is taught to be an important parameter for the properties of the adhesive composition. Although the releasable and readherable adhesive compositions disclosed in the documents discussed hereinbefore perform satisfactory, there is still interest in developing alternatives. One of the reasons for that is the presence of relatively large amounts of plasticizer and/or liquid tackifying resins in the prior art compositions. EP-A-0,443,263 teaches that these components are necessary for obtaining a composition having a low CMTg, which is essential for a good releasability of the adhesive composition. One of the objects of the present invention is to provide a releasable pressure sensitive adhesive composition containing a relatively small amount of plasticizer and/or liquid tackifying resin, while still exhibiting an excellent releasability in combination with sufficient tackiness, even after having been used repeatedly. Adhesive compositions containing a relatively small amount of plasticizer and/or liquid tackifying resin are known from EP-A-0,519,086. This European patent application discloses an adhesive tape, of which both sides are coated with an adhesive layer. One adhesive layer is made of an acrylic or rubber-based pressure sensitive adhesive composition and a relatively small amount of silicone polymer, while the other adhesive layer is solely constituted of an acrylic or rubber-based pressure sensitive adhesive composition. Due to the different composition of both layers, they can be easily released and separated from each other at their interface, thus enabling the adhesive tape to be stored or sold as a roll. The easy separation of both layers at their interface is attributed to the presence of the silicone polymer in one of the layers, which silicone polymer bleeds to the surface of the adhesive layer where it forms a silicone film. Due to its visoelasticity, this silicone film exhibits adhesive properties and hence can serve as an adhesive layer. Since the silicone polymers disclosed do not contain any reactive group or atom which might interact with the acrylic or rubber polymer molecules, it is clear that the silicone polymer does not act as a cross-linking agent for connecting the various acrylic or rubber polymer molecules present in the pressure sensitive adhesive composition. EP-A-0,340,979 discloses a composition comprising a pressure sensitive adhesive and a liquid organohydrogenpolysiloxane containing on the average at least two silicon-bonded hydrogen atoms per molecule. When this composition is combined with a composition comprising the same pressure sensitive adhesive and a curing agent for the said liquid organohydrogenpolysiloxane, a cross-linkable pressure sensitive adhesive composition is obtained. However, the adhesive properties are such that it cannot be used as a releasable adhesive. Hence, the use of the composition as a releasable adhesive is evidently not intended. The pressure sensitive adhesive may be based on styrene-butadiene random copolymers as well as on acrylic or butyl polymers. Styrene-butadiene block copolymer-based adhesives however, are not explicitly disclosed. Among the large group of suitable curing agents disclosed, there are mentioned platinum compounds, such as platinum acetylacetonate and chloroplatinic acid. As becomes apparent from the working examples of EP-A-0,340,979, particularly examples 34 and 35, such platinum compounds must be used in conjunction with a curing accelerator, such as methylvinylsiloxane, in order to achieve an acceptable level of cross-linking in styrene-butadiene polymer-based pressure sensitive adhesives. The present invention aims to provide a releasable pressure sensitive adhesive composition, which adheres very well to various substrates including paper and plastics, which can be removed from the substrate without leaving any traces behind and which can be used repeatedly without loss of tackiness. It is a further object of the present invention to provide a releasable pressure sensitive adhesive composition which contains a relatively small amount of plasticizing oil and/or liquid tackifying resin. More specifically, the present invention aims to provide a releasable pressure sensitive adhesive composition based on a monovinyl aromatic-conjugated diene block copolymer, wherein the block copolymer molecules are cross-linked via a cross-linking reaction with an organohydrogenpolysiloxane cross-linking agent without the use of any curing accelerator. It is also an object of the present invention to provide a releasable pressure sensitive adhesive composition, wherein the monovinyl aromatic-conjugated diene block copolymer is cross-linked through its vinyl groups present in the poly(conjugated diene) blocks. Surprisingly, it has now been found that releasable pressure sensitive adhesive compositions based on cross-linked block copolymers can be obtained without using any curing accelerator, which compositions contain relatively small amounts of plasticizer, i.e. less than 50 parts by weight per 100 parts by weight of block copolymer. Accordingly, the present invention relates to a releasable pressure sensitive adhesive composition comprising: (a) a block copolymer comprising at least one poly(monovinyl aromatic hydrocarbon) block and at least one poly(conjugated diene) block and having a vinyl content of at least 3%, (b) a solid tackifying resin, and (c) a plasticizer, wherein the block copolymer has been cross-linked through its vinyl groups by means of a reaction with an organohydrogenpolysiloxane cross-linking agent containing at least two hydrogen atoms being directly bonded to a silicon atom in the presence of a cross-linking catalyst and wherein the plasticizer is present in an amount of from 1 to 50 parts by weight per 100 parts by weight of block copolymer. The expression "vinyl groups" as used throughout this specification refers to the ethylenically unsaturated groups pending from the poly(conjugated diene) backbone portions of the block copolymer, which groups result from polymerization of the conjugated diene monomer at the 1,2- or 3,4-positions. Accordingly, the expression "vinyl content" refers to that portion of the conjugated diene monomer molecules incorporated in the block copolymer, which have polymerized at the 1,2- or 3,4-position. By using the existing functionality present in the poly(conjugated diene) blocks of the block copolymer in the form of vinyl groups, the cross-linking reaction can be performed in the presence of only a cross-linking catalyst without the use of any curing accelerator. Moreover, by using said existing functionality to establish cross-linking, a possible additional reaction for introducing functional groups, such as carboxyl groups or amine groups, is avoided. The block copolymer used in principle may be any block copolymer comprising at least one poly(monovinyl aromatic hydrocarbon) block A and at least one poly(conjugated diene) block B containing sufficient vinyl groups to enable an acceptable level of cross-linking. It will be understood that the expression "poly(monovinyl aromatic hydrocarbon) block" covers every polymer block containing polymerized monovinyl aromatic hydrocarbon as the main constituent. Accordingly, small amounts of other copolymerized monomers, i.e. up to 20% by weight but preferably less than 10% by weight, may be present as well. The same applies for the expression "poly(conjugated diene) block". The block copolymer may have a linear, radial or star-shaped structure. A preferred type of block copolymer however, is a linear ABA type block copolymer comprising two poly(monovinyl aromatic hydrocarbon) endblocks A and one poly(conjugated diene) midblock B. Such ABA block copolymer also includes AB'/ABA block copolymer mixtures with A and B as defined above and with B' representing a poly(conjugated diene) block having approximately half the apparent molecular weight of the polymer block B. Such mixtures result from the commonly applied coupling processes for preparing ABA block copolymers, wherein difunctional coupling agents are used for coupling two living AB' polymer chains prepared by anionic solution polymerization. The general concept of such processes is for instance disclosed in Chapter 3, paragraph 4 of N.R. Legge et al., THERMOPLASTIC ELASTOMERS, Carl Hanser Verlag, Munich (1987). In general, the final product resulting from a coupling process will always be an AB'/ABA block copolymer mixture which may contain up to 40% by weight of AB' block copolymer, the exact content of AB' block copolymer being determined by the coupling efficiency of the coupling agent used. When using conventional difunctional coupling agents, such as dibromoethane or diethyladipate, the finally obtained AB'/ABA mixtures contain 10 to 20% by weight of AB'. These AB'/ABA block copolymer mixtures are, however, still referred to as "ABA block copolymers". Another suitable type of block copolymer is a radial block copolymer of the general formula (AB) n (B) m X wherein A and B are as defined above, X is a coupling agent residue and n and m are integers of from 0 to 30. Other suitable types of block copolymers are those containing homopolymeric blocks as well as tapered polymer blocks, i.e. polymer blocks containing both monovinyl aromatic hydrocarbon and conjugated diene wherein the content of one of these monomers gradually increases in one direction. Beside AB'/ABA block copolymer mixtures obtained by the coupling method, AB'/ABA block copolymer mixtures obtained by physically blending an AB' and an ABA block copolymer in any desired weight ratio may also be used. It will be understood that in this case the apparent molecular weight of B' is not necessarily half the apparent molecular weight of B. The only essential criterium is that the conjugated diene polymer blocks B and B' contain sufficient vinyl groups to enable a sufficient level of cross-linking. For all types of block copolymers it is preferred that the poly(monovinyl aromatic hydrocarbon) block is a polystyrene block and the poly(conjugated diene) block is a polybutadiene or polyisoprene block. The content of conjugated diene in the block copolymer may vary within a wide range, but suitably should be in the range of from 65 to 95% by weight, preferably 75 to 90% by weight, based on total weight of block copolymer. Consequently, the content of monovinyl aromatic hydrocarbon in the block copolymer ranges from 5 to 35% by weight and preferably from 10 to 25% by weight. It will be understood by those skilled in the art that a too high content of monovinyl aromatic hydrocarbon results in a block copolymer which is too hard to be suitably applied in adhesives. The apparent molecular weight of the block copolymer, i.e. the molecular weight as determined by gel permeation chromatography using polystyrene calibration standards, may vary over a broad range and suitably lies within the range of from 25,000 to 1,000,000. For linear block copolymers the apparent molecular weight preferably lies between 50,000 and 300,000, while in the event of branched or star-shaped block copolymers the apparent molecular weight may even exceed 1,000,000. The vinyl content should be sufficient to enable an acceptable level of crosslinking upon reaction with the crosslinking agent. In general, the level of crosslinking mainly depends on the vinyl content and on the amounts of both crosslinking agent and catalyst used. It will be understood that a relatively low vinyl content may still result in an acceptable level of crosslinking, if the amount of crosslinking agent employed is sufficiently high to react with all or almost all of the vinyl groups. Accordingly, the vinyl content is not extremely critical and may vary within wide limits. In order to guarantee a minimum level of crosslinking a vinyl content of 3% is however required. The upper limit is not particularly critical and may be as high as 75%. In a preferred embodiment of the present invention, the vinyl content is in the range of from 3 to 60%, more preferably 4 to 40%. Since in general vinyl contents of 15% by weight or more require special measures to be taken during the polymerization process -as will be discussed below-, it is highly advantageous to employ block copolymers having a vinyl content of from 3 to 10% by weight. Such block copolymers namely, can be obtained by a polymerization process which does not require special measures for increasing the vinyl content of the block copolymers obtained. The vinyl content of the block copolymer is usually controlled by the addition of a structure modifier to the polymerization mixture when preparing the block copolymer. Examples of suitable structure modifiers are disclosed in DE-A-3,108,583 and include inter alia ethers, such as e.g. tetrahydrofuran, diglyme (diethyleneglycol dimethylether) and diethoxypropane, as well as tertiary amines, such as triethylamine. Other suitable structure modifiers are disclosed in EP-A-0,162,483 and EP-A-0,128,607. When no structure modifier is used, the block copolymer will normally have a vinyl content of between 3 and 10%. In general, the block copolymers may be produced by any well known polymerization process, such as e.g. disclosed in US-A-3,231,635; US-A-4,219,627 and US-A-4,391,949. Examples of commercially available block copolymers, which may be used in the adhesive composition according to the present invention, include CARIFLEX TR-1101, CARIFLEX TR-1102 (both linear styrene-butadiene-styrene block copolymers) and CARIFLEX TR-1107 (a linear styrene-isoprene-styrene block copolymer. CARIFLEX is a trade mark. An example of a suitable styrene-isoprene multi-armed block copolymer is KRATON D-1320X (KRATON is a trade mark). Suitable solid tackifying resins are those which are solid at room temperature and which are compatible with the poly(conjugated diene) polymer block(s). The tackifying resin may be present in amounts up to 400 parts by weight per 100 parts by weight of block copolymer (phr) and preferably is present in an amount of from 10 to 300 phr, even more preferably from 50 to 200 phr. Most preferably, the tackifying resin is present in an amount of from 100 to 150 phr. Combinations of two or more tackifying resins can also be suitably applied. Suitable poly(conjugated diene) compatible tackifying resins then include polyterpene resins, polyindene resins, rosin esters, hydrogenated rosins, alpha-pinene resins, beta-pinene resins, hydrocarbon resins of petroleum origin and phenolic resins. Examples are REGALITE R91, R101, R 125 and S260 (REGALITE is a trade mark), ESCOREZ 1310 and 5380 (ESCOREZ is a trade mark), WINGTACK 95 (WINGTACK is a trade mark), FORAL 85 and 105 (FORAL is a trade mark), PICCOLYTE A115, S115 and S10 (PICCOLYTE is a trade mark) and PICCOTAC 95E (PICCOTAC is a trade mark). Mineral oils, both naphthenic and paraffinic oils, or low molecular weight polybutylene polymers may be used as the plasticizer. Examples of suitable plasticizers are SHELLFLEX 371 and 451, CATENEX 956 and TUFFLO 6204 (naphthenic oils), TUFFLO 6056 (paraffinic oil) and the polybutylenes HYVIS 200, NAPVIS 30 and NAPVIS D-10. SHELLFLEX, CATENEX, TUFFLO, HYVIS and NAPVIS are trade marks. Also very useful are liquid resins, such as REGALREZ 1018 (REGALREZ is a trademark), and other oils, such as ONDINA 68 (ONDINA is a trade mark) and V-OIL 7047 (V-OIL is a trade mark). The plasticizer is used in amounts of from 1 to 50 phr, more preferably 5 to 40 phr and most preferably 10 to 35 phr. In addition to the solid tackifying resin and the plasticizer other additives, such as antioxidants, UV stabilizers, fillers, flame retarders and the like, may be present as well depending on the specific conditions under which the releasable pressure sensitive adhesive composition is to be used. As the antioxidant, any antioxidant commonly applied in adhesive formulations may be used. An example of such antioxidant is the phenolic compound IRGANOX 1010 (IRGANOX is a trade mark). The organohydrogenpolysiloxane crosslinking agent must contain at least two hydrogen atoms being directly bonded to a silicon atom, which can undergo a polyaddition reaction with the vinyl groups of the different block copolymer chains, thus effecting crosslinking. In principle, any organohydrogenpolysiloxane which meets this criterion may be suitably applied as the crosslinking agent for the purpose of the present invention. Suitable organohydrogenpolysiloxanes then include those listed in EP-A-0,340,979. Preferred organohydrogenpolysiloxanes are those having the general formula wherein R on each position independently represents a methyl or ethyl group, m is an integer of from 0 to 200, preferably 0 to 150, and n is an integer of from 2 to 60, preferably 10 to 50. Preferably, all groups R represent methyl. An example of a suitable commercially available crosslinking agent is DOW CORNING 7048 (DOW CORNING is a trade mark). Other organohydrogenpolysiloxane crosslinking agents may be sold by Wacker Chemie. As the crosslinking catalyst there can be used any catalyst known in the art to be suitable applied for catalyzing polyaddition reactions between SiH groups in the presence of active metal-based catalysts, particularly platinum- and rhodium-based catalysts. Specific examples of suitable catalysts are e.g. known from EP-A-0,340,979. For the purpose of the present invention platinum-based catalysts, such as platinum acetylacetonate, chloroplatinic acid (H 2 PtCl 6 .nH 2 O) and potassium or sodium salts thereof, are preferred. A useful commercially available platinum-based catalyst is DOW CORNING Q2-7403. Other platinum-based catalysts may be sold by Wacker Chemie and Rhone-Poulenc. The releasable pressure sensitive adhesive compositions according to the present invention can be very suitably applied as the adhesive strip or layer of removable self-adhering memo notes or as the adhesive component of removable tapes and labels. Another application may be the use in resealable closures, such as used for various kinds of packages of tissues and the like or such as used in diapers. It will be understood that any shaped article, such as removable tape, memo or label, comprising the pressure sensitive adhesive composition as hereinbefore described, also forms an aspect of the present invention. The invention is now further illustrated by the following examples, however without restricting the scope of the invention to these specific embodiments. Example 1 100 parts by weight of CARIFLEX TR-1107 (SIS block copolymer having a vinyl content of 5%, a styrene content of 14% by weight and an average total apparent molecular weight of about 208,000), 125 parts by weight of a tackifying resin (PICCOTAC 95E), 25 parts by weight of plasticizing oil (SHELLFLEX 451), 1 part by weight of antioxidant (IRGANOX 1010), 1 part by weight of organohydrogenpolysiloxane (DOW CORNING 7048) and 3 parts by weight of crosslinking catalyst (DOW CORNING Q2-7403) were dissolved in toluene, so that the final solution had a solid content of approximately 37% by weight. The solution was subsequently spread onto a polyester sheet with a bar coater, available from BRAIVE Industries. After evaporation of the toluene in an oven at 50 °C, an adhesive layer having a thickness of 22 µm was obtained. The adhesive layer was found to have the following properties: Rolling Ball Tack (RBT) >30 cm Looptack (LT) 14 N/25 mm 180° Peel Adhesion (PA) 10 N/25 mm SAFT 105 °C Example 2 and Comparative Example 1 In a similar way as described in Example 1 two other compositions were prepared and spread onto a polyester sheet. The compositions and the properties of the resulting releasable adhesive layer are listed in Table I. Adhesives with linear block copolymer Ex.2 Comp.Ex.1 CARIFLEX TR-1107 100 100 PICCOTAC 95E 125 125 SHELLFLEX 451 25 25 IRGANOX 1010 1 1 DOW CORNING 7048 2 0 DOW CORNING Q2-7403 6 0 RBT (cm) >30 2 LT (N/25 mm) 5 14 PA (N/25 mm) 5 11 SAFT (°C) 108 101 Examples 3 and 4 and Comparative Example 2 In a similar way as described in Example 1 three compositions were prepared and spread onto a polyester sheet. The block copolymer used was a star-shaped block copolymer containing styrene-isoprene (SI) block copolymer arms pending from a divinyl benzene (DVB) core. On the average, each molecule contains about 8 SI-arms. Styrene content of this block copolymer was 10% by weight, vinyl content 8% by weight and apparent molecular weight of each SI arm 110,000. The compositions and the properties of the resulting releasable adhesive layers are listed in Table II. Adhesives with star-shaped block copolymer Ex.3 Ex.4 Comp.Ex.2 (SI) 8 DVB 100 100 100 PICCOTAC 95E 125 125 125 SHELLFLEX 451 25 25 25 IRGANOX 1010 1 1 1 DOW CORNING 7048 1 2 0 DOW CORNING Q2-7403 3 6 0 RBT (cm) >30 >30 3 LT (N/25 mm) 11 1 14 PA (N/25 mm) 7 2 13 SAFT (°C) 108 109 108 Examples 5 and 6 and Comparative Example 3 In a similar way as described in Example 1 three compositions were prepared and spread onto a polyester sheet. The block copolymer used was a linear SIS block copolymer having a styrene content of 15% by weight and a vinyl content of 7% by weight. The compositions and the properties of the resulting releasable adhesive layers are listed in Table III. Adhesives with linear block copolymer Ex.5 Ex.6 Comp.Ex.3 SIS 100 100 100 PICCOTAC 95E 125 125 125 SHELLFLEX 451 25 25 25 IRGANOX 1010 1 1 1 DOW CORNING 7048 1 2 0 DOW CORNING Q2-7403 3 6 0 RBT (cm) >30 >30 3 LT (N/25 mm) 8 2 19 PA (N/25 mm) 9 4 11 SAFT (°C) 117 117 116 From the preceding examples it can be concluded that the adhesive compositions according to the present invention are more useful as releasable adhesives than the adhesive compositions not containing cross-linked block copolymer, while the short term heat resistance is about equal for both adhesive compositions.

Claims: A releasable pressure sensitive adhesive composition comprising: (a) a block copolymer comprising at least one poly(monovinyl aromatic hydrocarbon) block and at least one poly(conjugated diene) block and having a vinyl content of at least 3% by weight, (b) a solid tackifying resin, and (c) a plasticizer, wherein the block copolymer has been cross-linked through its vinyl groups by means of a reaction with an organohydrogenpolysiloxane crosslinking agent containing at least two hydrogen atoms being directly bonded to a silicon atom in the presence of a crosslinking catalyst and wherein the plasticizer is present in an amount of from 1 to 50 parts by weight per 100 parts by weight of block copolymer. A releasable adhesive composition according to claim 1, wherein the block copolymer is a linear block copolymer comprising two poly(monovinyl aromatic hydrocarbon) endblocks and one poly(conjugated diene) midblock. A releasable adhesive composition according to claim 1, wherein the block copolymer is a radial block copolymer of the general formula (AB) n (B) m X wherein A is a poly(vinyl aromatic hydrocarbon) block, B is a poly(conjugated diene) block, X is a coupling agent residue and n and m are integers of from 0 to 30. A releasable adhesive composition according to any one of the preceding claims, wherein the poly(monovinyl aromatic hydrocarbon) block is a polystyrene block and the poly(conjugated diene) block is a polybutadiene or polyisoprene block. A releasable adhesive composition according to any one of the preceding claims, wherein the organohydrogenpolysiloxane has the general formula wherein R on each position independently represents a methyl or ethyl group, m is an integer of from 0 to 200 and n is an integer of from 2 to 60. A removable self-adhering memo note having an adhesive strip or layer comprising the adhesive composition according to any one of claims 1 to 5. A removable tape or label comprising the adhesive composition according to any one of claims 1 to 5.

Inventors: DE KEYZER NOEL RAYMOND MAURICE, VERMUNICHT GEERT EMIEL ANNA, DE KEYZER, NOEL RAYMOND MAURICE, VERMUNICHT, GEERT EMIEL ANNA
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1994
Main classification: C09J153
IPC codes: C09J153, C09J125, C09J7, C08F8